Query DNS to find NAPTR in iOS - ios

Question

I have been having a lot of trouble finding a way to query the DNS to find the NAPTR in iOS. There seem to be many relatively simple ways to resolve to an IP, but I specifically need to find all NAPTR records in a DNS lookup. I'd prefer to do so without having to bring in any external libraries if at all possible. If anyone has been able to do this (or something similar that I can extrapolate from) I'd appreciate any pointers.
All code must function in iOS 5.0+

Answer 1

I ended up using DNSServiceQueryRecord.
DNSServiceRef sdRef;
DNSServiceQueryRecord(&sdRef, 0, 0,
"google.com",
kDNSServiceType_NAPTR,
kDNSServiceClass_IN,
callback,
NULL);
DNSServiceProcessResult(sdRef);
DNSServiceRefDeallocate(sdRef);
In actual use, I found that there was an issue where the app would hang indefinitely if there were no results, so I ended up having to adjust my code to add a timeout on the result.
/*
Attempt to fetch the NAPTR from the stored server address. Since iOS will continue waiting
until told directly to stop (even if there is no result) we must set our own timeout on the
request (set to 5 seconds).
On success, the callback function is called. On timeout, the kSRVLookupComplete notification
is sent.
*/
- (void)attemptNAPTRFetch {
dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
DNSServiceRef sdRef;
DNSServiceErrorType err;
err = DNSServiceQueryRecord(&sdRef, 0, 0,
[server cStringUsingEncoding:[NSString defaultCStringEncoding]],
kDNSServiceType_NAPTR,
kDNSServiceClass_IN,
callback,
NULL);
// This stuff is necessary so we don't hang forever if there are no results
int dns_sd_fd = DNSServiceRefSockFD(sdRef);
int nfds = dns_sd_fd + 1;
fd_set readfds;
struct timeval tv;
int result;
int stopNow = 0;
int timeOut = 5; // Timeout in seconds
while (!stopNow) {
FD_ZERO(&readfds);
FD_SET(dns_sd_fd, &readfds);
tv.tv_sec = timeOut;
tv.tv_usec = 0;
result = select(nfds, &readfds, (fd_set*)NULL, (fd_set*)NULL, &tv);
if (result > 0) {
if(FD_ISSET(dns_sd_fd, &readfds)) {
err = DNSServiceProcessResult(sdRef);
if (err != kDNSServiceErr_NoError){
NSLog(#"There was an error");
}
stopNow = 1;
}
}
else {
printf("select() returned %d errno %d %s\n", result, errno, strerror(errno));
if (errno != EINTR) {
stopNow = 1;
postNotification(kSRVLookupComplete, nil);
}
}
}
DNSServiceRefDeallocate(sdRef);
});
}
Then, for the callback:
static void callback(DNSServiceRef sdRef,
DNSServiceFlags flags,
uint32_t interfaceIndex,
DNSServiceErrorType errorCode,
const char *fullname,
uint16_t rrtype,
uint16_t rrclass,
uint16_t rdlen,
const void *rdata,
uint32_t ttl,
void *context)
{
uint16_t order, pref;
char flag;
NSMutableString *service = [[NSMutableString alloc] init];
NSMutableString *replacement = [[NSMutableString alloc] init];
const char *data = (const char*)rdata;
order = data[1];
pref = data[3];
flag = data[5];
int i = 7;
while (data[i] != 0){
[service appendString:[NSString stringWithFormat:#"%c", data[i]]];
i++;
}
i += 2;
while(data[i] != 0){
if(data[i] >= 32 && data[i] <= 127)
[replacement appendString:[NSString stringWithFormat:#"%c", data[i]]];
else
[replacement appendString:#"."];
i++;
}
NSLog(#"\nOrder: %i\nPreference: %i\nFlag: %c\nService: %#\nReplacement: %#\n", order, pref, flag, service, replacement);
}
This seems to do the trick for me. You would of course do any other necessary work using all the parsed data in the callback or store the data somewhere to be used later.